chore(CPlusPlus): add fibonacci (#703)

algorithms/CPlusPlus/Dynamic-Programming/fibonacci.cpp

@@ -0,0 +1,34 @@
+/*
+Fibonacci series- 0,1,1,2,3,5,...
+Given a number n, we have to print the nth term of the fibonacci series
+Example- if n=0, the answer will be 0
+         if n=1, the answer will be 1
+         if n=1, the answer will be 1 (sum of 0 and 1)
+         if n=2, the answer will be 2 (sum of 1 and 1)
+         and so on..
+
+Time Complexity - O(n)
+Space Complexity - O(1)
+*/
+
+class Solution {
+public:
+    int fib(int n) {
+        
+        if(n<2)        // if n<2, i.e, if n is 0 or 1 return n because 0th term is 0 and 1th term is 1
+            return n;
+        
+        int prev1=1;  
+        int prev2=0;
+        
+        for(int i=2; i<=n; i++)
+        {
+            int curr = prev1+prev2;  // the current term is sum of the two terms before it, here prev1 is (i-1)th term and prev2 is (i-2)th term
+            prev2=prev1;
+            prev1=curr;
+        }
+        
+        return prev1;  //at the end prev1 will have the value of the nth term, so we will return it
+        
+    }
+};

algorithms/CPlusPlus/README.md

@@ -38,6 +38,7 @@
 - [0/1-knapsack](Dynamic-Programming/01-knapsack.cpp)
 - [Matrix chain Multiplication](Dynamic-Programming/matrix-chain-multiplication.cpp)
 - [Edit Distance](Dynamic-Programming/edit-distance.cpp)
+- [Fibonacci](Dynamic-Programming/fibonacci.cpp)
 
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